This invention relates to a method for detecting abnormality in sensor means for detecting a parameter relating to a quantity of intake air being supplied to an internal combustion engine, as well as for effecting a fail safe function in the event of occurrence of such abnormality.
A fuel supply control system adapted for use with an internal combustion engine, particularly a gasoline engine has been proposed e.g. by U.S. Pat. No. 3,483,851, which is adapted to determine the fuel injection period of a fuel injection device for control of the fuel injection quantity, i.e. the air/fuel ratio of an air/fuel mixture being supplied to the engine, by first determining a basic value of the above valve opening period as a function of engine rpm and intake pipe absolute pressure and then adding to and/or multiplying same by constants and/or coefficients being functions of engine rpm, intake pipe absolute pressure, engine temperature, throttle valve opening, exhaust gas ingredient concentration (oxygen concentration), etc., by electronic computing means.
In such fuel supply control system, when an output from a parameter sensor for detecting the value of a parameter relating to the operating condition of the engine shows an abnormal value due to disconnection in the wiring system of the parameter sensor, etc., the fuel injection quantity is not controlled to proper values, badly affecting the operation of the engine such as driveability, if the control of the fuel injection quantity is continued on such occasion by using such abnormal value as an actual value of the parameter. Therefore, a fail safe measure is necessary which ensures continued operation of the engine even in the event of occurrence of a failure in parameter sensor means, i.e. a failure in the sensor body itself, the wiring system. To comply with such necessity, a method has been proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 54-141926, which comprises assuming that an abnormality occurs in a parameter sensor when an output from the parameter sensor shows a value lying outside a predetermined normal output value range, and then applying a predetermined value of the parameter to control of the fuel supply quantity to the engine, in place of an actual output value from the parameter sensor.
However, such abnormality detecting method cannot be applied to a case where an output from the parameter sensor shows a value lying within the predetermined normal output value range even when an abnormality actually occurs in the parameter sensor. More specifically, for instance, in the event that a pipe connecting an intake pipe pressure sensor with the intake pipe of the engine becomes disconnected from either the pressure sensor or the intake pipe, the pressure in the intake pipe does not act upon the pressure sensor and consequently the output from the pressure sensor shows a value constant relative to the atmospheric pressure irrespective of actual changes in the pressure in the intake pipe. If in such event the constant output value lies within a normal output value range which can take place during normal operation of the engine, it is impossible to detect such abnormality in the parameter sensor means as caused by disconnection of the pipe connecting the pressure sensor with the intake pipe.